The present invention relates to the field of acoustical holography and, more particularly, to an improved liquid interface area detector for effecting acoustical imaging.
Acoustical holography, which involves the generation of holograms through the production of interfering patterns of acoustical waveforms, has been known for some time. One early example of an apparatus for practicing acoustical holography is disclosed in U.S. patent application Ser. No. 569,914 -- Brenden, filed Aug. 3, 1966 and entitled "Ultrasonic Holography", now U.S. Pat. No. 3,879,989. In this patent means are shown for producing a pattern of perturbations at the interface of two fluids in response to the impingement of a pair of ultrasonic beams. The wave trains of each beam give rise to irregularities upon the liquid interface, the pattern of perturbations representing the phase relationships between various portions of the beams. By using one unmodulated beam as a reference, the pattern resulting from interference between it and a beam whose wavefront phase is spatially modulated will represent the characteristics of the modulation of the second beam.
Modulation of an acoustic beam may, for example, result from the presence of an object of varying acoustical transparency in the path of the beam. The interference pattern will then represent variations in the acoustical transmissivity of the object. In this manner a representation can be produced of the inner structure of an optically opague object, such as flaws or voids within a metal casting or the arrangement of tissue within the human body.
In order to record or display the interference pattern given rise to by the reference and the object beams, it is known to illuminate the pattern with a beam of visible light. The light which is reflected from the pattern, hereinafter referred to as a hologram, is then directed to appropriate apparatus for recording or displaying the hologram.
While the science of acoustical holography has made substantial progress in recent years, many problems still exist in satisfactorily implementing the technique.
With one prior art approach, for example, a reference and an object beam are directed from beneath upon a detection device floating in an acoustically transmissive liquid. An illuminating beam of light is directed upon the top of the detector. In such an arrangement multiple reflections of acoustical energy within the detector structure degrade the quality of the hologram.
The detectors to be used with imaging systems have themselves presented substantial problems. In particular, many of the detectors heretofore known have been relatively inefficient, i.e., much of the acoustic and/or optical energy directed toward the detector was lost as a result of inefficient or unwanted reflection or due to absorption. In addition, most detectors, especially those utilizing a liquid-liquid interface, are particularly susceptible to extraneous vibrations. Such vibrations tend to cause spurious perturbations at the liquid interface, thus disturbing the pattern of the hologram and seriously affecting its quality. Further, most prior art liquid interface detectors required the passage of either the impingent light, the acoustic beams, or both through a window or membrane either before or immediately after encountering the hologram.
It is therefore an object of the present invention to provide an improved detector for acoustical holography apparatus.
It is another object of the present invention to provide a detector for use in acoustical holography which provides improved reflectivity to illuminating radiation.
It is still another object of the invention to provide a detector for use in acoustical holography which exhibits markedly improved stability against vibration.